The rex gene of phage lambda is enigmatic in having no discernable function but unique control of expression. It and the adjacent cI repressor gene are the only lambda genes active in a lambda lysogen. We shall investigate rex gene function by three genetical-physiological approaches: 1) rex mutants have slight defects in lysogeny, tending to form unstable lysogens. We shall try to relate this deficiency to a role of rex in control of the int xis genes of the prophage. 2) Certain colicin-tolerant bacterial mutants (especially tolB) interfere with exprersion of the lambda rex gene in a lambda lysogen. We shall continue our physiology characterization of several classes of tol mutants to develop models for the function of these genes. 3) We shall develop a model for the ecological role of prophage genes that are expressed in lysogenic bacteria. It will be based on observations about lambda rec gene expression under specialized growth conditions and will be extrapolated to the case of animal viruses.